Answer to Question #71181 in Optics for Vaibhav vashisth

Question

The magnifications produced by a convex lens for two different positions of an object are m1 and m2 respectively (m1>m2) if "d" is the distance of seperation between the two positions of the object then focal length of the lens is
(1) √m1m2 (2) d/√m1-m2 (3) dm1m2/m1-m2 (4) d/m1-m2 correct answer is d/m1-m2 but how??